As described in U.S. Pat. No. 8,049,182 issued to Bewick on Nov. 1, 2011, (hereinafter “Bewick”) hereby incorporated by reference in its entirety (however, where anything in the incorporated reference contradicts anything stated in the present application, the present application prevails) with reference to FIG. 1, for X-ray analysis in an electron microscope (EM) 100, an X-ray spectrum is measured by sensing and measuring the energies of individual X-ray photons emitted by a sample 101 when it is hit by a focused electron beam 102. Each X-ray photon is an energetic particle and the energy is typically converted into charge using an X-ray detector 105. Electrons which are scattered back from the sample, so called “backscattered electrons” (BSE) 103, may also travel towards the X-ray detector 105. An electron with the same energy as an X-ray photon deposits the same amount of energy in the X-ray detector 105 and therefore gives a similar signal charge. Typically, the number of electrons travelling towards the detector is considerably larger than the number of X-ray photons, and therefore the signal due to electrons represents a large proportion of the measured X-ray spectrum and can overwhelm the contribution due to X-ray photons. The BSE contribution in the spectrum is typically a large background extending over all energies up to the primary beam energy. X-ray analysis may be conducted in principle in any apparatus using an incident beam of charged particles (e.g., electrons or ions) and although the description herein refers to electrons, the same principles apply in any charged particle apparatus.
The X-ray detector is usually isolated from the vacuum of the EM by a thin foil of typically polymer supported on a grid with high transparency. If BSE strike the foil or the grid, they generate X-ray signals characteristic of the materials in the foil or grid and these X-ray signals appear as a spurious contribution to the recorded X-ray spectrum. As a result of the incident beam, the X-ray photons are emitted in all directions and the X-ray detector only detects photons falling within a cone defined by the active area of the sensor within the X-ray detector. The higher the solid angle defined by this cone, the more signal is collected and this is highly desirable. The solid angle can be increased by increasing the active area of the X-ray detector, and so a large sensor and large aperture are beneficial.
The solid angle can also be increased by locating the sensor closer to the sample. However, when the sensor is placed closer to the sample 101, the tube structure of the X-ray detector 105 containing the sensor can collide with the final pole piece 104 of the electron microscope 100. This collision is shown by way of example at point A in FIG. 1.
Therefore, it is desirable to minimize the external diameter of the detector tube (shown at B in FIG. 1) to minimize the distance between the detector face and the sample without colliding with the conical pole piece of the electron microscope.